The present invention relates to optical signal technology. More specifically, the present invention relates to optical sampling of polarized signals.
Optoelectronics technology and its applications are expanding with the result that integrated optics technology can be used with considerable advantages in communications. In these communication systems, optical signals embody information bits at very high bit rates, for example, 160 Gigabits per second. Such optical signals are often sent via single mode optical fibers. When, an optical signal is received as an input signal, the bits of the input optical signal are sampled and converted to information. In the input optical signal, each bit may be represented an optical pulse having a waveform.
For measuring the waveforms of optical pulses used in high bit rate optical communications, it is common practice and desirable to use optical sampling with high sensitivity and high time resolution. Optical sampling systems often use a probe pulse signal and optical mixing with a user signal to achieve what is known as sum frequency generation (SFG) that is very useful for obtaining representations of sampled user signals. Some implementations of the sum frequency generation processes may use a nonlinear crystal such as, for example, a periodically poled lithium niobate (PPLN) crystal. See, for example, xe2x80x9cHighly Sensitive and Time-Resolving Optical Sampling System Using thin PPLN Crystalxe2x80x9d by S. Nogiwa, et al., Electron Lett, Vol. 36, IEEE 2000.
The PPLN and other optical devices operate most efficiently when its optical input signal has a particular polarization direction. However, the polarization direction of the input signal is difficult to determine. Moreover, the polarization direction of the input signal changes over time. Hence, the efficiency of the PPLN can swing widely from very high (for example, near 100 percent) when the polarization of the input optical signal is aligned with that of the PPLN to very low (for example, near zero percent) when the polarization of the input optical signal is orthogonal to that of the PPLN. Such unpredictable swings in sampling efficiency are undesirable. In fact, the unpredictability of the polarization of input optical signals and polarization direction changes of the input optical signals over time introduces difficulties in analysis of the input optical signal.
Accordingly, there is a need for a method and apparatus to minimize the difficulties associated with unpredictability of the input signal polarization.
These needs are met by the present invention. According to one aspect of the present invention, a split waveplate is disclosed. A first half of the split waveplate has refractive index along a first direction and a second half of the split waveplate has refractive index along a second direction.
According to a second aspect of the invention, a split half-waveplate has a first half having refractive index along a first direction and a second half having refractive index along a second direction, the second direction being 45 degrees relative to the first direction.
According to a third aspect of the invention, a method of forming a split waveplate is disclosed. First, a waveplate having a directional refractive index is cut at 22.5 degrees off the direction of the refractive index to produce a first and a second portion of the waveplate. The second portion is flipped. Then, the split waveplate is formed by joining the first portion and the flipped second portion along the cut such that the direction of the refractive index of the flipped second portion is 45 degrees off from the direction of the refractive index of the first portion.
According to a fourth aspect of the invention, an apparatus for sampling optical input signal includes a split waveplate for spatially rotating polarization direction of a first portion of the input signal to a first rotated direction and spatially rotating polarization direction of a second portion of the input signal to a second rotated direction orthogonal to the first rotated direction. Further, a sum frequency generator generates sum frequency of the rotated input signal.
According to a fifth aspect of the present invention, a method of sampling optical input signal is disclosed. First, the input signal is spatially rotated such that one half of the power of the input signal is within a first polarized portion of the input signal while the other half of the power of the input signal is within a second polarized portion of the input signal, the second polarized portion being orthogonally polarized relative to the first polarized portion. Then the spatially rotated input signal is sampled.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.